Offshore vessel, especially those of the semisubmersible type, are often equipped with thruster propellers, which are rotatable 360.degree. and are used for propulsion, steering as well as for positioning of the vessel. Mounting and dismounting of these thruster propeller units is a difficult and tedious work.
As the propeller units are mounted below the bottom plating of the vessel, the space within the latter, adjacent to the location of mounting, must be separated from the surrounding water. An occasional screening around the opening, where the propeller unit is to be mounted, can easily be arranged, but the lines used for performing the terminating part of the lifting of the unit into the opening has hitherto caused many problems.
Usually there is a vertical tube for each line, which from a hole in the bottom plating, within the vessel, reaches up to or somewhat above the water level. As the decks of the pontoons carrying the semisubmersible vessel during a mounting operation will be located just above water level, the space remaining for the lifting gear will be rather restricted. The required space for the lifting gear must be provided above the deck of the pontoon, in localities needed for other purposes, for instance as bunker or ballast tanks.